Influence of Prunus domestica gum on the release profiles of propranolol HCl floating tablets

Propranolol hydrochloride is a beta-blocker used for the management and treatment of hypertension, angina, coronary artery disease, heart failure, fibrillation, tremors, migraine etc. The objective of the present study was to design Propranolol Hydrochloride floating tablets by direct compression method and to explore the role of a new gum as a matrix former. A 22 full factorial design was selected for the present study. Prunus domestica gum and HPMC (K4M) were used as independent variables, swelling index and drug dissolution at 12 hours as dependent variables. Formulations were subjected to pre- and post-compression tests that showed good micromeritics and buoyancy characteristics (Carr’s index 11.76%–14.00%, Hausner’s ratio 1.13°–1.16°, angle of repose 22.67°–25.21°, floating lag time 56–76 seconds, total floating time 18–25 hours and swelling index 59.87%–139.66%). The cumulative drug release in 0.1 N HCl at 12 hours was 72%–90% (p<0.05). Weibull model was found to be the best fit model (R2>0.99) among all other studied models. Multiple regression showed a significant effect of Prunus domestica gum and HPMC K4M on the swelling index and dissolution profiles of propranolol HCl (p<0.05). On the basis of better in-vitro performance and cost-effectiveness, formulation F4 was the best formulation. It is evident from the results that Prunus domestica gum possesses excellent drug release retardant potential for the floating drug delivery system and this new gum should be further explored alone or with other natural and synthetic polymers in future studies.


Introduction
Oral drug delivery is preferred over other routes of drug administration because it offers higher patient compliance, ease of administration and variety in the formulation of dosage non-toxic, less expensive and easily available compared to synthetic polymers [23]. They have multiple pharmaceutical applications such as binder, disintegrant, suspending and emulsifying agents. The controlled release pharmaceutical formulations have also been developed from the natural gums such as guar gum, karaya gum and xanthan gum [19,[23][24][25]. Similarly, Prunus domestica gum has been introduced recently and reported as an effective tablet binder in the formulation of oral dosage forms [26,27]. However, to the best of our knowledge, it has not been studied for controlled release pharmaceutical application. Therefore, the present study was designed to explore this gum as a matrix former in FDDS.

Gum extraction.
The gum was extracted from the plant Prunus domestica using the method reported by Rahim et al with slight modification [28]. The extracted gum was dried at 50-60˚C for three hours in a tray dryer. The dried gum was hydrated (in 500ml water) for 8 hours followed by straining through muslin cloth to remove any extraneous material. It was then precipitated by adding 10mL of absolute acetone to the solution. The precipitates were separated and dried overnight at 50˚C in a tray dryer. The pure gum thus obtained was stored in a tightly closed container at room temperature for future use as a matrix former in GRFT.

Formulation of floating tablets.
A 2 2 full factorial design was employed in the current study with two factors at two levels (low and high) to determine the influence of factors, variables and their interactions at both levels. Prunus domestica gum and HPMC K4M were taken as independent variables while swelling index and percent drug dissolution (at 12 hours) as dependent variables (Tables 1 and 2). All the ingredients were accurately weighed in polybags. Propranolol HCl (API) and Avicel pH 102 were taken in a mortar and mixed by geometric dilution. The pre-blend was sieved through 30 mesh and transferred into a polythene bag, other formulation excipients (except magnesium stearate and talc) were also sieved through 30 mesh, added to the mixture and blended for 20 minutes to get a homogeneous mass. Finally, magnesium stearate and talc (passed through 80 mesh) were added to the polybag and the blend was further mixed for 5 minutes by tumbling. The powder blend was kept in a tightly closed container before micromeritic evaluation and compression. A single punch machine with 8 mm round punches was used for tablet compression (Fig 1). The tablet weight was set at the target weight of 250 mg/tablet.

Micromeritic properties of the floating swellable tablets.
Bulk and tapped densities were determined by taking approximately 2 g powder blend in a 100mL graduated cylinder. Bulk volume was noted by tapping the cylinder twice while tapped volume was noted until no further change in volume was observed after the continuous tapping. The respective densities were calculated by -1 and -2 [29]. The data thus obtained were used for the calculation of Carr's Index (CI) and Hausner's Ratio (HR) by -3 and -4 [30]. The angle of repose was calculated by the fixed height method. A funnel was fixed on a stand at a height of 1cm above the hard flat surface. The powder was poured through the funnel and a cone of the powder blend was formed on a paper. The height of the cone and radius of the powder blend were noted and the angle of repose was calculated by -5 [29].
2.2.4. Physical tests. Weight variation, thickness, diameter, hardness and friability tests were conducted. Twenty tablets were selected randomly and the weight of individual tablets was noted using a weighing balance. The thickness, diameter and hardness of 10 of the selected tablets were measured using Vernier Caliper and Pfizer hardness tester. Upper and lower control limits along with mean and standard deviation (SD) were calculated. The friability test was performed on 20 randomly selected tablets for 4 minutes at the rotational speed of 25 rpm (100 rotations) in a Roche Friabilator. The tablets were shaken to remove the excess powder and cleaned with a soft brush. The de-dusted tablets were weighed and their friability was calculated by -6.

In-vitro buoyancy studies. The floating Lag Time (FLT) and
Total Floating Time (TFT) of each formulation were determined. Five tablets were randomly taken in a separate beaker, 500 mL 0.1 N HCl solution. The FLT was noted after the tablets started to rise on the surface while the TFT was noted by determining the time tablets remained floating on the surface [31]. Similarly, the Swelling Index (SI) was determined on 3 tablets selected individually from each of the formulations. The tablets were weighed (W i ) and poured into a beaker containing a 100 mL solution of 0.1 N HCl maintained at 37˚C. After certain time tablets were removed from the beaker and their surface was cleaned with tissue paper to remove the excess liquid. The final weight was determined, and SI was calculated by -7 [32].
2.2.6. Assay of the drug. Ten tablets were taken randomly and finely powdered in a mortar with a pestle. A sample equal to 40mg of crushed tablets was taken into a 50ml volumetric flask, 20 ml of 0.1N HCl was added and shaken vigorously till dissolved. The final volume was made up to the mark with the same solvent. The absorbance of the sample was measured by a UV double beam spectrophotometer at a 289 nm wavelength [5].
2.2.7. In-vitro dissolution study. The dissolution was performed in a USP type-II dissolution apparatus using 900ml of 0.1N HCl as a dissolution medium. The temperature of the media was maintained at 37˚C±0.5˚C. Around 5ml aliquot was withdrawn at specified time intervals i.e., 0.5, 1, 2, 4, 6, 8, 10 and 12 hours from the vessel and media was replenished with the same volume to maintain the sink condition. The samples were filtered and absorbance was taken at 289 nm by UV spectrophotometer [5]. The percent drug dissolution was calculated by -8.
The in-vitro drug release data were fitted into different kinetic models such as zero order, first order, Higuchi, Korsmeyer Peppas, Hixon Crowell and Weibull models to determine the mechanism of drug release. The data were also compared by similarity (f1) and dissimilarity (f2) factors. Moreover, multiple regression (MR), is considered to be an effective tool for generating equations that predict the relationship between the independent and dependent variables [33] One-way ANOVA was applied to the responses to find the relationship between the two variables.

Micromeritics of trial formulations
Bulk and Tapped densities ranged from 0.43 to 0.46 g/cm 3 and 0.50 to 0.53 g/cm 3 respectively. These values are closer to each other which indicates non-significant inter-particulate interactions among the formulations excipients good packing characteristics, important for storage, flow and compaction of the powder during manufacturing [34]. These results were further verified by corresponding Carr's index and Hausner'sratio which were 11.76 to 14.00% and 1.133 to 1.162% respectively. To ascertain flowability, the angle of repose was determined that ranged from 22.67 to 25.21˚respectively. This data indicates excellent flow and good compressibility characteristic of the powders blends (Table 3) [35,36]. This ensured minimum batchto-batch variation and uniform content distribution of all formulation blends which is a prerequisite for satisfactory compression [37].

Characteristics of the tablets
All formulations were compressed without any tableting defects. The average weight of tablets ranged from 249.95±2.39 mg to 250.85±3.00 mg. The hardness ranged from 7.77±0.45 kg/cm 2 to 8.84±0.57 kg/cm 2 . The hardness was found favorable for the floating tablets as evident from a previous study [38]. The percentage friability was found within the pharmacopeia limit (< 1%) in all trial formulations. All results followed the official standards and were essential for robust tableting characteristics. The trial formulations showed API content within the range (97-99%, p>0.05) and thus complied with BP specifications that require 90-110% API in the composite samples (Table 4). This indicated appropriate blending and uniform random mixing of all the formulation ingredients.   Table 4). The sodium bicarbonate was used as a gas-generating agent in present formulations. It has been reported that it reacts with water and generates carbon dioxide gas (Fig 2). The resulting gas entraps within the polymer matrices and ultimately cause a decrease in the overall density of the formulation. This enables the formulation to float upon the gastric media [39]. The highest FLT was observed in the formulation F1 which contained the highest amount of polymer and gum (HPMC 110 mg and gum 75 mg). Conversely, formulation F4 contained the lowest amount of polymer and gum (HPMC 60 mg and gum 50 mg) exhibited the lowest FLT. In the case of TFT, the formulation F2 (containing a lower amount of polymer) floated for around 18 hours in tested media while the formulation F1 (containing a higher amount of the polymer) around 25 hours. Thus, the rapid disintegration of the tablets is evident from formulation F2. It appears that the viscosity of the polymer and the corresponding, amount in the formulation,  might be considered to be associated directly with the FLT and TFT of the floating tablets [39]. Nevertheless, it is apparent from the results that when the concentration of gum and polymer increased, both FLT and TFT also increased and vice-versa.

Swelling index.
A gradual and uniform swelling index was observed in all the formulations (Table 5). However, the formulation F1 showed the highest swelling index 77.23 ± 1.722% (2 hours) to 139.66 ± 1.44% (12 hours) and formulation F4 showed the lowest swelling index 59.87 ± 1.28% (2 hours) to 110.76 ± 1.33% (12 hours) respectively. Again the concentration of polymers appeared to be directly related to the swelling of the tablets [20,40].

In-vitro drug release studies
The cumulative drug release showed minimum drug release from the formulation F3 followed by F1, F4 and F2 respectively (Fig 3). In the first hour, F1, F2 and F4 formulations appeared to be closer and superimposable except for F3. Then a decline in cumulative drug release was observed in formulations F1 and F3 compared to F2 and F4 and this pattern continued till the last sampling point. The behavior in drug release might be due to the high concentration of polymer in the formulation which formed a strong gel layer around the tablet and thus made the diffusion layer stronger that consequently resulting in a slower release rate. This leads to slow erosion and hindrance in the diffusion of API from matrix tablets (Nigusse et al., 2021). It has been revealed that the formulation F4, containing a lower amount of polymer and gum, showed the initial burst release of Propranolol HCl. A slower drug release was observed in formulation F3. This could be attributed to the binding effect of Avicel PH-102 and the higher concentration of polymer. It is evident from the literature that a higher concentration of polymer is associated with a stronger diffusion layer. This produces a formulation more resistant to erosion and diffusion [41]. However, the cumulative drug release profile of formulation F1 was comparatively better than formulation F3. Nevertheless among all formulations, a gradual and uniform drug release was observed in formulation F4 for 12 hours. It appears that both polymer and gum had

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produced an appropriate gelling effect that maintained the matrix integrity and resulted slowly with sustained release of the drug till 12 hours.

Release kinetics
The Weibull model showed the highest determination coefficient (R 2 = 0.9974-0.9999) followed by Korsemeyer Peppas, Higuchi, First Order, Hixon Crowell and Zero Order respectively (Table 6). Formulations F3 and F4 showed β <1 exhibiting a parabolic curve whilst formulations F1 and F2 showed a sigmoidal curve with β > 1 [20]. The diffusion and erosion of matrix tablets upon hydration were well explained by the Korsmeyer Peppas model. The 'n' specifically determined the drug release mechanism from the polymeric system [28].
Only formulation F4 followed Fickian diffusion drug release (n � 0.5) while the rest of the formulations F1, F2 and F3 showed non-Fickian drug release (n >0.5 but <1.0) [42,43]. Moreover, the dissolution data were analyzed by f1 and f2 factors. The factors were applied using formulation F2 or F4 as a reference due to their comparable drug release profile as attributed by paired t-test (p<0.05) to whom all other formulations were compared. The formulations were accepted and rejected based on statistical acceptance criteria i.e., the mean values (Mean R and Mean T). The range 0 to 15 for f1 and 50 to 100 for f2 were considered acceptable as recommended for pairwise comparison [44,45]. Based on these criteria, the formulations F1 and F2 were found similar to F4, or the formulations F1 and F4 were found similar to F2 while the formulation F3 was found dissimilar in both the cases (Figs 4 and 5).

Optimization of propranolol floating tablets by multiple regression method
Multiple regression (MR) is considered an effective tool for generating prediction equations that provide suitable measurement [33]. The responses of Prunus domestica gum and HPMC K4M on the swelling index and drug dissolution obtained in the present study were subjected to MR analysis to find the relationship between the independent and the dependent variables and to optimize the best propranolol HCl floating tablet.
The regression statistics showed an R 2 of 0.884 for the swelling index after 12 hours of floating of the propranolol HCl formulation. The data was analyzed by ANOVA which detected a significant difference in the swelling index of all trial formulations (p<0.05). Both gum and polymer resulted in a positive value of regression coefficient i.e. 0.45 and 0.35 respectively. It indicates a direct relationship between gum/polymer and swelling index. It means that when the concentration of either gum/polymer increased, the swelling index also increased as verified and depicted in line fit plots (Figs 6 and 7).
The regression statistics showed an R 2 of 0.994 for cumulative drug dissolution of propranolol HCl after 12 hours of sampling time. A higher value of R 2 indicates a close relationship among the dissolution profiles of all trial formulations. However, when analyzed statistically, a significant difference in the dissolution profiles was detected by ANOVA (p<0.05). Furthermore, the regression coefficients were 0.364 for gum and -0.1820 for HPMC K4M respectively. Contrary to the swelling index where both variables were positive, here we get an opposite effect of both variables on the drug dissolution profile. The positive value of gum showed a direct relationship of gum concentration on the dissolution whilst the negative value of HPMC

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K4M indicated an inverse relationship between the polymer concentration and dissolution. Both relationships were further confirmed by line fit plots (Figs 8 and 9). Although the formulations F2 and F4 showed comparable characteristics, the formulation F4 might be declared as the best formulation based on low consumption of gum and polymer (making it cost-effective), analysis of multiple regression and other desirable physicochemical attributes of FDDS as described earlier.

Conclusions
The influence of Prunus domestica gum as a matrix former in floating tablets was studied for the first time. A floating time of more than 25 hours was successfully obtained using the gas formation technique. The drug release mechanism showed non-Fickian diffusion except for the formulation F4 which confirmed a Fickian diffusion. The formulation F4 could be considered the best formulation because it contained a lower concentration of gum and polymer (cost-effective) and met all the desirable attributes targeted in the present study. Thus Prunus domestica gum appeared to be a suitable candidate for the formulation of FDDS. The formulation will help minimize the limitations related to oral therapy as well as reduce therapy cost, improves

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patient compliance and make production easy due to the high natural availability, low cost and nontoxic nature of the excipient. Its role alone or in combination with other synthetic and natural polymers should be explored in future studies. To further determine its usefulness as a new matrix former in the formulations of FDDS and other controlled release formulations.